1. Technical Field
The present invention relates generally to an improved solder flux and paste useful in microelectronic applications and in particular, to an improved water soluble solder flux and paste. Still more particularly, the present invention provides an improved water soluble solder flux and paste that is thermally stable.
2. Description of the Related Art
In producing printed circuit boards, both primary attachment and the rework/repair of components for these printed circuit boards have been accomplished utilizing rosin-based solder pastes and rework/repair fluxes. These organic, rosin systems require non-aqueous solvents, such as chlorofluorocarbon ("CFC") solvents, for proper cleaning after the attachment or rework/repair of components.
Solder paste contains a flux as a component. Solder powder is suspended in the flux. This flux component of the solder paste is the material that controls many of the requirements of the solder paste. For example, the flux must remove metal oxides from the solder powder and other metallurgy present to allow reflow of all the solder properly.
In a typical manufacturing operation, solder paste is screened onto a circuit board, using an appropriately patterned stencil, in all of the locations requiring connection of components to copper pads. Next, components are placed on the circuit board on the prepared sites. The solder paste must be tacky enough to hold the components in their required location until the board is subject to reflow, at which time the solder paste flows to become the soldered connection for the components. Reflow is the term used to describe heating the solder above its melting temperature to give a soldered joint.
The rosin based solder pastes currently utilized in the industry have the major part of the flux system composed on a complex mixture derived from a natural product based on pine-tar residue (called rosin), which results when volatile fractions are distilled from the pine-tar. As mentioned above, such rosin-based flux systems require an organic solvent to clean the flux residue after reflow. Although the best solvent for cleaning such fluxes is a halogenated hydrocarbon, the current desire to decrease the use of halogenated hydrocarbons, makes the use of this type of flux less desirable. Recently, in a global effort to curtail CFC emissions, water-soluble solder pastes and water-soluble rework/repair fluxes have been introduced which may be cleaned sufficiently with aqueous cleaners to produce a residue-free solder joint.
The presently available water soluble solder fluxes and pastes are sensitive to thermal changes and certain levels of thermal energy. Thus, the presently available water soluble solder fluxes and pastes are sensitive to the various conditions encountered during storage and transportation of the water soluble fluxes and pastes. In particular, the thermal stability of the water soluble solder paste, produced with solder powder and a water soluble flux, is a very common shortcoming of the presently available water soluble pastes. For example, some water soluble solder pastes, i.e., WSP 1206 or 1208 from Alpha Metals, Incorporated, must be stored at very cold temperatures in order to prevent various components of the water soluble solder paste from separating and/or undergoing an undesirable chemical change. The Alpha Metals product, WSP 1206 or 1208, has a marked decrease in performance if the product is not kept continuously frozen until utilized. In other cases, the water soluble solder paste is sensitive to temperature changes encountered during shipping. If a water soluble solder paste, normally, stored at ambient temperatures encounters low temperatures, i.e., zero degrees centigrade, some of the components may precipitate or drop out of the mixture to form crystals, making the paste unusable. Expensive measures are thus necessary to maintain the integrity and ensure the quality of the presently available water soluble solder fluxes and pastes.
Therefore, it would be desirable to have a water soluble solder flux and paste that is stable at various conditions encountered prior to use including a wide range of temperatures.
It would also be desirable to provide such a water soluble flux and paste which has the desired physical and performance characteristics required for microelectronic applications but without the attendant disadvantages of rosin-based systems.
It would also be desirable to provide such a flux and paste which is economical to manufacture and which can be formulated from commercially available components.